In the decommissioning of nuclear PowerStation's there is a need to treat wet wastes. These wastes are made up of solids such as sands from old filter beds, ion exchange resins, activated carbons and wastes from laundries and wash waters. The particles are allowed to settle over time in ponds, which are specially constructed tanks, such as final delay and monitoring tanks. This creates a sludge layer at the bottom of the tank, which has to be safely removed and transported to a cementation process to encapsulate this radioactive waste for safe disposal.
GB 0212728.0 describes a fluidising unit which creates a swirling flow and fluidises solids, such as waste sludge, and can be used to transport these solids at the required relative density to a cementation or sinter process, or other means of disposal, such as boxes/drums for burial in specially constructed deep mines, or bore holes in the earth's strata. The disclosures of that document are incorporated herein by reference.
U.S. Pat. Nos. 4,978,251, 4,952,099, 4,992,006 and 5,853,266 disclose fluidising units which could also be used for the transportation process.
In practice the type of sludge that is encountered tends to have a very high proportion of small particles, typically in the range 0.1 microns to 30 microns. These particles can also have a structure based on interwoven platelets of clay for example, which have a high porosity, which together with their size renders them almost neutrally buoyant in water, or at the very least very slow settling.
The rate at which these solids will settle can be calculated using Stokes Law assuming variables such as density of solids, water temperature, and viscosity of water, shape of solids particle, expected particle interactions and concentrations, which are all known and based on the normal force of gravity. This is better known as the particles hindered settling velocity and can typically be measured in part of a millimeter per second. Given therefore that the particle size distribution is known, based on size, volumes and counts per milliliter, a reasonable estimate can be made on the amount of time that will be required for the particles to settle out over a certain distance in a certain time, normally referred to in separation as required retention time.
Based on this the size of a tank or pressure vessel receiving the slurry of fine solids can be physically sized to ensure that no solids are carried over out of the top of the receiving vessel due to displacement of water due to settling solids or pump suctions. Such a vessel could be prohibitively large and uneconomically viable, as well as requiring too much time to effect separation or inhibit carry over of solids.
Patent Specification CA2007250 shows a system for accepting slurry for hydrotransportation by using a fluidising unit, which uses an inclined torroidal shaped vessel constructed from pipes with 180° returns on each of its ends, where the overflow water, displaced by incoming solids passes through a laminar type plate separator in an attempt to inhibit fine solids from carrying over. In practice the fine solids are too small to be effectively removed from the overflow water by this means.
With regard to the application discussed for decommissioning of nuclear PowerStation's this would be unacceptable, as the overflow waters would be returned back to the tank being cleaned, hence recycling all the fine solids back where they originated from; the need to eventually clean the water in the tank would be compounded by this return of solids, as the water would normally be pumped to a membrane filter, RO unit or molecular sieve, all of which would suffer operational problems when high levels of solids are delivered to them as their primary function is to act as tertiary or polishing filters. Any decanted solids from these units would have to report back to the transport vessel in some safe way to be delivered to, for example, the cementation process.